1. Technical Field
The disclosure relates to a cholesteric liquid crystal film and an article employing the same, and in particular relates to a broadband cholesteric liquid crystal film and an article employing the same.
2. Description of the Related Art
Cholesteric liquid crystal film is quite a unique optical element exhibits selective light reflection and circular dichronism. It is widely applied in various display devices or photoelectric elements, such as polarizer films or backlight modules of LCDs.
Cholesteric liquid crystal owns a layer-by-layer molecular stacking structure. The long axis directions of the molecular layers of the stacked molecular structure are in essence the same, Said molecular directors are twisted a small angle from one layer to another one layer. The helical pitch of a cholesteric liquid crystal is defined as the distance between liquid crystal molecular layers which are located at an angle of 360 degrees to each other along the long axis direction. The stacked molecular structure of a cholesteric liquid crystal layer can be of left-handed orientation or right-handed orientation depending on the chirality of the chiral compounds of the cholesteric liquid crystal. Specifically, a circular polarized light having the same circular dichronism as the cholesteric liquid crystal layer will be reflected, and the other circular polarized light having an opposite circular dichronism will pass through the cholesteric liquid crystal film.
According to the Bragg reflection rule, the relationship between wavelength (λ) of a cholesteric liquid crystal, an average refraction ratio (nave) of a thin film, and a helical pitch (P) of a cholesteric liquid crystal molecule is:λ=nave·P 
The reflection bandwidth (Δλ) relates to the birefringence (Δn) and helical pitch (P) of cholesteric liquid crystal molecules, according to the relationship equation:Δλ=Δn·P 
According to this formula, the selective reflection bandwidth of common cholesteric liquid crystal is strongly related to its birirefringence (Δn) and only about several tens of nanometers (such as 40-50 nm). It is only part of range of visible light, which covers bandwidth from 400 to 700 nm. It is thus important to improve the assembly structure of cholesteric liquid crystal molecules to meet the requirements for a wider selective reflection bandwidth.
Since the selective reflection bandwidth of a common single layer of a cholesteric liquid crystal film is limited, a plurality of layers of a cholesteric liquid crystal film are prepared by coating by multiple times such that the bandwidth of each layer can be combined together so as to cover the whole wavelength range of visible light.
For example, U.S. Pat. No. 6,016,177 discloses a multilayer cholesteric liquid crystal film 10 including layers 12, 14, and 16 of cholesteric liquid crystal polymers having different helical pitches, wherein the layers 12, 14, and 16 are combined by adhesive layers 18, as shown in FIG. 1. However, a plurality of manufacturing processes is required and there are multiple alignment interferences at the interfaces which adversely affect the polarizing effect thereof. Furthermore, the manufacturing process of the multilayer cholesteric liquid crystal film 10 is complicated, and the yield is reduced thereof.
In order to solve the aforementioned problems, a method for fabricating a multilayer cholesteric liquid crystal film via continuous coatings is provided. Due to the absence of the adhesive layers, the multilayer cholesteric liquid crystal film exhibits improved light transmittance. The layers of the multilayer cholesteric liquid crystal film, however, have similar surface tensions, which may cause coating defects within the multilayer cholesteric liquid crystal film.
On the other hand, since a single-layer cholesteric liquid crystal film has a simplified manufacturing process and less coating defects resulting from continuous coatings, a single-layer cholesteric liquid crystal film is desired to replace the conventional multilayer cholesteric liquid crystal films.
U.S. Pat. No. 5,506,704 discloses a single-layer cholesteric liquid crystal film. The variation of the helical pitch is achieved via a long-term UV exposure. Thereby, the helical pitch in the thickness direction is varied progressively. However, a the long time UV exposure is needed will be problematic for mass production, especially for low production efficiency, and specialized equipment needed.
U.S. Pat. No. 7,311,952 discloses a liquid crystalline film with broadened reflection bandwidth fabricated via a two-stage polymerization method. First, a coating of a polymerizable liquid-crystalline material is partially polymerized via an actinic radiation in an environment, which has an inhibiting action on the polymerization (such as air or air enriched with oxygen), to form a semi-polymerized structure. After a briefly re-alignment period, the semi-polymerized film is fully polymerized via a high-energy actinic radiation to form a soiled film. Referring to FIG. 2, the obtained cholesteric liquid crystal film 20 can be derived into three regions according to the helical pitch. The region 22 adjacent to the air top surface 21 of the cholesteric liquid crystal film 20 has the shortest helical pitch, the region 24 disposed between the region 22 and the region 26 has the longest helical pitch, and the region 26 has the middle value of the helical pitches in between the region 22 and region 24 does. The method, however, has an inherently high degree of process uncertainty due to the additional alignment time and harsh reaction conditions (such as radiation energy) of the partial polymerization.